1. Field of the Invention
The present invention relates to an improved form of bicycle speed change mechanism for bicycles and particularly to a speed change device in which there is an adjusting mechanism provided.
2. Description of the Prior Art
A conventional speed change mechanism as shown in FIG. 1 is well known wherein a speed change mechanism 100 is mounted at the rear of a bicycle, a torsion spring being mounted in a speed change support 101 of the speed change mechanism 100 to press a flange thereof against a baffle angle of the bicycle frame. When the rider pulls all adjusting cable 102 to change the riding speed, a double connecting lever sliding mechanism, installed on the speed change mechanism 100, produces a parallel sliding action and moves a chain guider 103 connected to the other end of the double connecting lever horizontally. A bicycle chain 104 is then moved to the next rear transmission gear accordingly. During this procedure, as the radius of the rear transmission gear changes, the whole speed changer must move backwards to compensate the difference resulting from such change so as to keep the bicycle chain 104 at the same tension; the force necessary for maintaining the chain tension comes from the torsion spring which is installed in the speed change support 101.
FIG. 2 shows the structure of the common speed change mechanism, in which a support shaft 206 is installed in a support body 203 and then fixed in a tapped hole 200a on the bottom end of a bicycle frame 200. Also mounted on the support shaft 206 there are a cover 205, a spring 204, a sleeve tube 202 and a C-type snap ring 201, in which the outer rim of the cover 205 is fixed at the end of the support shaft 206, the step-type outer rim of the other side of the cover 205 presses the spring 204 into the support body 203 and onto the sleeve tube 202. The right hand end bent part of the spring 204 is fixed in a groove 203a of support body 203, the left hand end bent part of the spring 204 being fixed into a guide hole 202b formed on the left hand side stepped part of the sleeve tube 202. A threaded part 206b of the support shaft 206 extrudes from the sleeve tube 202, and a C-type snap ring 201 is put onto the gap 206a to prevent the sleeve tube 202 from dropping out of support body 203 and the thread 206b of the support shaft 206 is locked into the tapped hole 200a of the bicycle frame 200. However, to change the riding speed, a bicycle rider must pull the adjusting cable very often to move the chain guide horizontally so as to move the chain to the next rear transmission gear. After a long period of use, the elastic fatigue phenomenon will easily occur in the torsion spring 204. As it is impossible to adjust the spring coefficient or modify the spring's torsion force, the time necessary to change riding speed becomes longer and longer; furthermore, the tension force of the chain maintained by the chain guide becomes weaker. These are the common defects which occur in usual bicycle speed change mechanisms.